netdev - Re: bpf pointer alignment validation

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Date:   Mon, 8 May 2017 10:30:20 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     ast@...com, daniel@...earbox.net, netdev@...r.kernel.org
Subject: Re: bpf pointer alignment validation

On Fri, May 05, 2017 at 10:47:09PM -0400, David Miller wrote:
> From: David Miller <davem@...emloft.net>
> Date: Fri, 05 May 2017 16:20:44 -0400 (EDT)
> 
> > Anyways, I'll play with this design and see what happens...
> > Feedback is of course welcome.
> 
> Here is a prototype that works for me with test_pkt_access.c,
> which otherwise won't load on sparc.

the approach looks good.

> +static u32 calc_align(u32 imm)
> +{
> +	u32 align = 1;
> +
> +	if (!imm)
> +		return 1U << 31;
> +
> +	while (!(imm & 1)) {
> +		imm >>= 1;
> +		align <<= 1;
> +	}
> +	return align;
> +}

who about
static u32 calc_align(u32 n)
{
	if (!n)
		return 1U << 31;
        return n - ((n - 1) & n);
}
instead?